The proliferation of new integrated circuit chip technologies has changed the requirements of printed circuit substrates in the electronics industry. In particular the use of leadless ceramic chip carriers results in 3 to 9 times higher packing density of operating components and the consequently more severe thermal management problems. Leadless ceramic chip carriers are designed to be surface mounted and the solder connecting the device to the printed circuit board is both an electrical and a mechanical connection. When these chip carriers are mounted on conventional circuit boards such as epoxy glass the mismatch in thermal expansion coefficients of the chip carrier and the board is significant. The chip carrier has a thermal coefficient of expansion (TCE) of about 6.4 ppm/.degree.C. over the temperature range of -55.degree. C. to 200.degree. C. while the epoxy glass has a TCE of 16 ppm/.degree.C. over the same temperature range. This high mismatch results in solder-joint stress failure during thermal cycling.
Moreover, the higher packaging density achievable with chip carriers generates more heat per unit area of printed circuit board. This heat must be dissipated to prevent high temperature failures in the devices. Conventional epoxy glass printed circuit board materials are thermal insulators and are not suitable in high packing density applications without separate provision for heat dissipation.
Several attempts to solve the problem have been made. Workers in the art have used a copper/iron-nickel alloy/copper sandwich construction as described in "Implementation of Surface Mount Technology in High Reliability Products", G. L. Horton, presented at National Electronic Packaging and Production Conference, NEPCON WEST, February 1987, Anaheim Convention Center, Anaheim, CA and in "Military Moves Headlong Into Surface Mounting", Special Report by Jerry Lyman, Electronics, July 10, 1986. In this configuration the TCE of the composite sandwich construction can be made to match the TCE of the leadless ceramic chip carrier, i.e. around 6.4 ppm/.degree.C. This iron-nickel alloy (Invar, also sold as NILO.TM. 36 by Inco Alloys International, Inc.) in the center of the sandwich has a TCE of 1.6 ppm/.degree.C. over the temperature range of -18.degree. C. to 175.degree. C. while that of the copper is 17 ppm/.degree.C. By placing 20% of copper on each side of the core Invar, the TCE can be held to around 7 ppm/.degree.C. over the aforespecified temperature range (unless otherwise stated thermal expansion mentioned hereinafter and in the claims are over this range of temperature).
However, this sandwich construction has one major drawback. While the copper has an excellent thermal conductivity of about 400 W/m..degree.C. the Invar has a thermal conductivity of only around 9.6 W/m..degree.C. This means that while the thermal conductivity along the strip is good, the conductivity through the strip is very poor. Thus the sandwich construction of copper/Invar/copper is not overly advantageous in advanced circuit board design.
Another approach to producing a material with controlled expansion properties and improved thermal conductivity through the sheet is described in U.S. Pat. No. 4,158,719 (the '719 patent). This patent teaches the blending of a highly thermal conductive powder such as copper with a controlled expansion alloy powder, compacting the blended powders, sintering at high temperature and cold rolling to produce a final product. All of the material in the '719 patent was sintered at a temperature of 982.degree. C. or higher. The composite strip material produced by this process was designed for lead frames.